1. Field of the Invention
The present invention relates to a receiver for use with a TDM (time-divisional multiplex) or TDMA (time-divisional multiplex access) mobile communications system employing digital modulation, and in particular to signal reception timing control.
2. Description of the Prior Art
Generally speaking, in TDM or TDMA radio systems the selective reception of a required signal from a time-divisional multiplex channel wherein it has been allocated a unit time slot and multiplexed demands that the reception time be synchronized with the time slot. With a digitally modulated signal it is also necessary for demodulation to be performed with a timing which is synchronized with the symbol timing of the signal. In receivers adapted to process digitally modulated signals, reception time is controlled in such a way as to fulfil these conditions, and this reception time synchronized with the received signal is referred to as reception timing.
As is shown in the block diagram in FIG. 1, the conventional method of reception timing control as described above involves the provision of a timing error detection means 3 upstream from the demodulation means 4 that demodulates the received signal, so that errors between signal time slot reception time and reception timing can be detected, and the reception timing of a given time slot is controlled in the reception timing control means 1 by the error output from the timing error detection means 3 at the time of reception of the preceding time slot.
With this conventional method of reception timing control, the timing of a given time slot is determined solely by the immediately preceding time slot. In the absence of any difference in arrival path between a given time slot and the preceding time slot, this method does not give rise to any substantial error between reception timing and the timing of the given slot. However, the existence of multiple arrival paths (multipath) means that there may be a substantial difference in the arrival paths of the given time slot and the preceding time slot, in which case a considerable error will occur between reception timing and the timing of the given time slot. This leads to poorer communication quality as a result of reduced accuracy of demodulation. In this way, conventional methods of reception timing control are susceptible to deterioration in quality in multipath communication.